1. Field of the Invention
The present invention relates to a bridge type ball screw and an electrically powered steering device having the ball screw.
2. Description of the Prior Art
The electrically powered steering device of an automobile is an instrument to assist the steering force of a steering wheel by means of an electric drive motor and is currently available in various types. One of the types employs is of a design wherein a retractable steering shaft coupled with a steering mechanism for vehicle wheels is imparted an axially shifting force that is transmitted thereto from the steering wheel through a motion translating mechanism such as a rack-and-pinion mechanism and, also, an axially shifting force that is transmitted thereto from an output of the electric drive motor through a ball screw. The ball screw that is compact and has high load capacity is desired so as to downsize a whole electrically powered steering device thereby to reduce weight of a whole automobile.
The ball screw is available in various types according to the manner in which a series of balls are circulated and one of the types is called a bridge type.
FIGS. 6(A) and 6(B) illustrate an example of the prior art ball screw mechanism of the bridge type. A screw shaft 51 has an externally threaded groove 52 in its outer face. A rotary nut 53 has an internally threaded groove 54 corresponding to the externally threaded groove 52 and is screwed on the screw shaft 51 through a number of balls 55 accommodated between both the threaded grooves 52 and 54. In a cylindrical body portion of the rotary nut 53, fitting holes 56 which are in oval shapes when viewed in a radial direction are formed to notch portions of the internally threaded groove 54 and to pass through inner and outer peripheral faces of the cylindrical body portion and oval bridge members 57 are fitted in the fitting holes 56. The bridge member 57 is a part having a connecting groove 58 for connecting portion s of the internally threaded groove 54 that are neighboring in an axial direction of the rotary nut 53 and substantially a lap of the internally threaded groove 54 and the connecting groove 58 define a ball guide passage for the balls 55. The large number of balls 55 arranged between the internally and externally threaded grooves 54 and 52 in the ball guide passage roll along the threaded grooves 54 and 52, are guided by the connecting groove 58 of the bridge member 57, climb over the thread of the screw shaft 51, and return to the neighboring internally threaded groove 54 to circulate.
The bridge type ball screw has an advantageous in that the rotary nut 53 can have a small outer diameter but has a disadvantage in that the number of component parts is large because the large number of bridge members are required. Moreover, the following problems are encountered when the load capacity is increased.
There are the following means of increasing the load capacity in the ball screw, but it is difficult to employ any of them in the bridge type ball screw as described below.
(1) A screw thread pitch is reduced to increase the number of ball circulations. If this means is employed and a ball diameter is reduced, the load capacity. is reduced on the contrary. Therefore, it is necessary to reduce the pitch without changing the ball diameter. However, because one bridge member 57 is necessary for one pitch (an interval between the threaded grooves) in the bridge type, a large number of bridge members 57 are required when the pitch is reduced without changing the ball diameter, which is difficult to realize in view of space.
(2) A groove is formed between leads to form multi-thread grooves to obtain similar effects to that in a case of increased number of circulations. In the multi-thread grooves, however, the lead (distance which the nut travels with a turn) is limited and the lead cannot be reduced. Although the multi-thread grooves can be employed for an end cap type, the multi-thread grooves cannot be employed for the bridge type because the bridge type requires one bridge member 57 for every pitch as described above.
Therefore, in order to increase the load capacity in the bridge type ball screw, there is no other choice but to increase a length of the nut and the number of the ball circulations. In this case, there is an advantage of reduced nut diameter but it is difficult to make the ball screw because the length of the nut is increased. As the length of the nut is increased, the number of the bridge members 57 is increased and the number of man-hours and the number of parts are further increased, which results in a raise in cost.
It is an object of the present invention to provide a ball screw in which the number of parts is small, which is compact, and in which a high load capacity can be obtained.
It is another object of the invention to provide an electrically powered steering device in which a ball screw for transmitting output of an electric drive motor is compact, a load capacity of the ball screw can be increased, and a whole device can be made compact.
A ball screw of the present invention comprises: a screw shaft; a rotary nut having at an inner periphery thereof inner spiral grooves facing the screw shaft; a plurality of balls disposed in a rolling guideway formed between the screw shaft and the inner spiral groove for transmitting a force between the rotary nut and the screw shaft; and a bridge member fitted to the rotary nut and having a plurality of connecting grooves. Each connecting groove connects neighboring convolutions of the inner spiral groove.
With this structure, since the plurality of connecting grooves are provided to one bridge member, the pitch of the inner spiral groove can be reduced while the bridge member being disposed with no difficulty in terms of the space available therefor. As a result, it is possible to increase the number of balls to be circulated and the capacity load without increasing the length of the rotary nut. Together with an advantage in that the diameter of the nut can be reduced similarly to the prior art bridge type ball screw, it is possible to form the ball screw that is compact and has high load capacity. Moreover, because the plurality of connecting grooves are provided to one bridge member, the number of the bridge members can be small and hence, the number of the parts can be reduced thereby facilitating machining process and the cost can be reduced. Furthermore, because the plurality of connecting grooves are provided to one bridge member, accuracy in forming the connecting grooves can be improved easily.
In this invention, the bridge member may be provided with an arm that is engaged with the inner spiral groove of the rotary nut so as to position the bridge member in an axial direction with respect to the rotary nut. The arm may be provided with a slit extending from a tip end toward a base portion of the arm.
As described above, because the arms of the bridge member are engaged with the inner spiral groove formed as a ball raceway of the rotary nut, the bridge member can be positioned with high accuracy. In case the arm is provided with the slit, the arm can have elasticity and can be fitted smoothly with an inner surface of the inner spiral groove without a gap therebetween.
In this invention, the bridge member may be provided with a pair of guide walls protruding radially outwardly of the rotary nut from opposite edges of the bridge member confronting to each other in a circumferential direction of the rotary nut. The guide walls may be in elastic contact with a pair of inner surfaces, facing to each other, of a bridge member fitting opening formed in the rotary nut.
As described above, by providing the bridge member with the guide walls, it is possible to fix the bridge member to the bridge member fitting opening while bringing the bridge member into elastic contact therewith and accordingly, the bridge member can be fitted easily.
In this invention, the bridge member may be inserted into the bridge member fitting opening formed in the rotary nut from a radially inner side. By inserting the bridge member from a radially inner side in this manner, any detach preventing means can be dispensed with.
In the case of providing the guide walls as described above, since the bridge member is arranged so as to be inserted from the radially inner side, the pair of guide walls may be provided at tip ends thereof with detach preventing projections to engage with opening edges on a radially outer side of the bridge member fitting opening. Since by inserting the bridge member into the bridge member fitting opening from the radially inner side can the detach preventing projections at the tip ends of the guide walls be engaged with the radially outer edge of the bridge member fitting opening, the bridge member can be fixed easily only by insertion work.
The guide wall may be provided with a slit extending from a tip end toward a base portion side of the guide wall. By providing the slit in this manner, elasticity of the guide wall can be increased thereby to easily fit the bridge member in the bridge member fitting opening with a smaller gap therebetween.
The bridge member may be made of a sintered alloy. If the bridge member is made of the sintered alloy, the bridge member can be produced by formation such as injection molding and sintering, which does not require machining such as turning or grinding and which has satisfactory mass productivity. Therefore, manufacturing at low cost can be attained.
An electrically powered steering device of the present invention comprises: a housing; a steering shaft connected to a steering mechanism for steering wheels; a motion translating mechanism for translating a rotary force exerted by a steering wheel into a force required to move the steering shaft in a longitudinal direction; a ball screw including a screw shaft constituted by a portion of the steering shaft; and an electric drive motor mounted on a rotary nut of the ball screw, wherein the ball screw is constituted by the ball screw with any above-described structure.
With this structure, because the ball screw for transmitting the electric drive motor output can be made compact while having high load capacity, the whole electrically powered steering device can be fabricated in a compact size.